Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Although many graphics professionals turn to Windows or Mac OS to execute their designs, Linux is far from helpless in this area. While it helps that Adobe Photoshop, the undisputed gold-standard program that most professionals use for raster graphics, runs on Linux through Wine, there are several native Linux programs that offer some of the same functionality. Furthermore, there are many free vector graphics programs that can produce infinitely scalable graphics much like what Adobe Illustrator can do. Aside from the software situation, there is no reason why Linux could not be just as effective with graphics applications as OS X and Windows, since Linux supports many peripherals like tablets out of the box with full plug-and-play support.
Are the Linux programs drop-in replacements for Photoshop and Illustrator? The answer could be either yes and no, depending on the way you look at it and what your needs are. If you compare the Linux alternatives to Photoshop/Illustrator feature-by-feature, the free open source tools will come up short by a significant margin and there is simply no way to get around that fact. If you actually need those features on a day-to-day basis, then you should get your wallet out and purchase Photoshop and/or Illustrator. However, if you can get by with less, the free open source software tools may be enough to get the job done and save you considerable money in the process.